An electrophotographic image forming apparatus includes an optical scanning device that emits a light beam to form an electrostatic latent image on an image carrying member. In this type of optical scanning device, the light beam emitted from a light source is scanned in a predetermined direction by an optical scanning member such as a polygon mirror. Subsequently, the light beam scanned by the optical scanning member is reflected by one or more reflection members that are elongated along the scanning direction of the light beam, and is guided to the image carrying member. Opposite ends of the reflection members in the longitudinal direction are supported by support portions of a housing of the optical scanning device.
Meanwhile, when the support portions are deformed due to a temperature change, angles of reflection surfaces of the reflection members may change. When this happens, the position of the light beam scanned on the image carrying member may be shifted in the sub scanning direction that is orthogonal to the scanning direction of the light beam on the image carrying member. In particular, in an image forming apparatus of a so-called tandem type including a plurality of image forming units, a color shift in the sub scanning direction may occur on a color image which is formed as a plurality of toner images formed by the image forming units are overlaid with each other. On the other hand, there is known a configuration where an end portion of the reflection member on a large temperature change side in the longitudinal direction of the reflection member is supported at one point, and an end portion of the reflection member on a small temperature change side is supported at a plurality of points (see, for example, PTL 1).